Currently a need exists for alternative hydrocarbon fuels, especially aviation and diesel fuels, from domestic sources to enhance energy security and to decrease reliance on foreign petroleum. Current routes to alternative fuels are limited by strict fuel standards and limited fuel feed stocks. And, many fuel and fuel blend stocks require open-chain hydrocarbons including, e.g., normal paraffins and branched paraffins. For example, according to ASTM D7566-11a standards, hydrogenated HEFA/SPK (Hydroprocessed Esters and Fatty Acids/Synthetic Paraffinic Kerosene) from bio-derived fats and oils and from Fischer-Tropsch reactions of syngas can only contain a maximum of 0.5% aromatics and 15% cycloparaffins. In addition, an average JP-8 jet fuel contains 59% normal and iso-paraffins, the remainder being aromatics and cycloparaffins. However, many conventional methods for producing alternate fuels such as from biomass feedstocks cannot meet these requirements. For example, pyrolysis and hydrothermal liquefaction of terrestrial biomass feedstocks form aromatics predominately and, when hydrotreated, yield cyclic hydrocarbons. Ethanol and other oxygenated hydrocarbons are suitable for direct blending with gasoline, but oxygenated hydrocarbons are precluded for use in jet fuels. Ethanol can be converted to liquid hydrocarbons over solid acid catalysts only at temperatures above about 300° C., but the products are largely aromatics (75%-90%). Thus, aviation and military organizations can expect difficulties meeting renewable fuel standards for jet fuels and diesel fuels using conventional technologies. Yet, ethanol is available in the market place. Thus, converting ethanol to oxygen-free open-chain hydrocarbons could permit their use in diesel and jet fuels. More specifically, catalytic conversion of renewable ethanol to oxygen-free open-chain hydrocarbons could allow for production of renewable fuels from oxygenated renewable feedstocks, such as carbohydrates and lignocellulosic biomass.
Ethylene is a feedstock available from numerous sources that could be converted to alternate open-chain hydrocarbon fuels. Ethylene can be obtained from sources such as natural gas, coal, and petroleum. Ethylene is also obtainable by known technologies from ethanol, which in turn can be made from biomass-derived sugars and starch and from syngas. Ethanol therefore can be considered an ethylene precursor. However, conversion of ethylene via conventional direct, single step conversion processes catalyzed by solid acid catalysts, such as silicoaluminates, is typically characterized by high process temperatures (>280° C.) that form large quantities of coke, and extensive formation of aromatic compounds up to 70 wt %. Single-step processes such as that reported by Heveling et al. over Ni/Si—Al and other catalysts are reported to produce open-chain hydrocarbons at high ethylene conversions, but with selectivities to ≧C10 of only ca 40% and to ≧C8 of only about 63%. Further, multi-step conversion processes reported in the literature have potentially better selectivities to open-chain compounds, but conversions to date are low and significant quantities of aromatic compounds are produced. For example, Synfuels International reports a multi-step process using Ni catalysts at process temperatures from 220° C. to 240° C. that produces a product composition containing between 4% to 90% aromatics. At the reported maximum selectivity of 70% middle distillate products and an ethylene conversion of only 26%, the maximum possible product yield in the middle distillate range is only about 18%. The 2-step Synfuels International process does not improve upon and, in fact, gives a lower distillate yield (18%) than the 1-step process reported by Heveling (40%). Thus, the 2-step approach by Synfuels International does not represent an economically feasible approach for obtaining high yields of distillate fuels. Accordingly, new processes and catalysts are needed that convert ethylene obtained, e.g., from various ethanol feedstocks into suitable oxygen-free hydrocarbon fuel blend stocks that minimize the production of aromatic hydrocarbons and the quantity of hydrogen needed to produce fuels, and that produce fuel precursors and/or fuel blend stocks that maximize flexibility in blending ratios suitable for production of jet fuel, other aviation fuels, diesel, and heating fuels. The present invention addresses these needs using, surprisingly, a 2-step method that provides distillate yields greater than the 18% of the prior art.